1. Field of the Invention
This invention relates to information transceivers. In particular, this invention relates to multi-carrier information transceivers with radio frequency interference reduction.
2. Description of Related Art
Multi-carrier communications transceivers allow the high-speed transmission of information using the twisted-pair telephone lines that connect individual subscribers to a telephone central office. Each pair of copper wires provides a communication channel in which the frequency response attenuates as the frequency increases. The wires also contain noises of a different nature produced by a variety of sources. Among these noises are thermal noises produced by electric devices and cross-talk noises produced by, for example, other subscribers connected to the same central office and sharing the same bundle of twisted-pairs.
The twisting of the twisted-pairs help to reduce the cross-talk noise by limiting electromagnetic coupling between the pair of lines that are close together. However, as the frequency of operation increases, the effect of twisting is limited and the cross-talk noise increases proportional to frequency.
In order to provide reliable communications over a channel with limited bandwidth and frequency-dependent noise, multi-carrier transceivers apply a xe2x80x9cdivide and conquerxe2x80x9d strategy. In this strategy, the total bandwidth of the communication channel is divided into a number of frequency sub-bands. Each sub-band is a sub-channel in which an information signal is transmitted. The width of the frequency sub-bands is chosen to be small enough to allow the distortion introduced by a sub-channel to be modeled by a simple complex value representing the attenuation and phase shift of the received signal. Various information signals are transmitted simultaneously using the various sub-channels. The receiver is able to separate the information signals in the different frequency sub-bands by using a bank of band-pass filters each one tuned to one of the different sub-bands. If these filters are chosen properly, the noise in each frequency band can be modeled using only the noise level present in that sub-band, with the noise in one band having little to no effect in the adjacent sub-bands.
A primary advantage of a multi-carrier transceiver is that the transceiver parameters can be optimized for different channel conditions in order to obtain maximum performance. The optimization process can be summarized as follow: First, a desired bit error rate is established. Second, the signal-to-noise ratio available in every sub-channel is measured. The bit error rate and the signal-to-noise ratio are then used to determine the maximum bit transmission rate that the sub-channel can support. Finally, an optimal set of information signals capable of transmitting this maximum bit transmission rate is found. By optimizing each sub-band, the total transmission capacity of the multi-carrier transceiver for a given error rate is maximized.
Usually, the noise in the telephone lines also contains radio frequency interference (RFI) produced by, for example, electromagnetic coupling of radio frequency signals coming from radio broadcasting transceivers that operate in the same radio frequency band as the multi-carrier transceiver. When present, this RFI can degrade the performance of the multi-carrier transceiver significantly, making the multi-carrier transceiver operate well below its optimum performance. The nature of the RFI is different from the difficulties associated with thermal noise and crosstalk noise. Optimizing a transceiver to operate in the presence of all the noises results in transceivers with great complexity, such as the transceiver disclosed by Sandberg et al. in 1995 entitled xe2x80x9cOverlapped Discrete Multitone Modulation for High Speed Copper Wire Communications.xe2x80x9d In practice, RFI mitigation techniques that minimize the degradation in performance are preferred.
For ease of illustration the following terminology will be used to discuss the operation of an exemplary multi-carrier transceiver. Specifically, an idle channel is a communications channel that may contain noise, crosstalk and RF signals in any portion of the spectrum, but does not contain upstream or downstream multi-carrier signals. The carriers in the multi-carrier transceiver will be denoted as tones. A tone is disabled when there is no energy transmission in that particular tone. A training or initialization signal, which is typically sent during the training state, is a multi-carrier transceiver initialization training signal used to train the transceiver before commencing the transmission of information. For the multi-carrier transceiver known as ADSL, these training signals are defined in the INITIALIZATION section of ITU standards G.992.1 (G.dmt), G.992.2 (G.lite) and the G.994.1 (G.hs), incorporated herein by reference in their entirety.
Steady state signals or information signals are the signals sent by the multi-carrier transceiver when communicating information data bits. The steady state transmission typically follows the training state transmission. For the multi-carrier transceivers known as ADSL, the steady state signals are defined in the SHOWTIME sections of ITU standards G.992.1 (G.dmt) and the G.992.2 (G.lite), incorporated herein by reference in their entirety.
An RFI band is a group of one or more tones in which a single RFI is identified. In general, the location of these bands within the total bandwidth of transmission is not known until the operation of the multi-carrier transceiver starts; and the tones in an RFI band may or may not be disabled during the transceiver operation. However, there are certain restricted RFI bands where the presence of RFI is highly probable. The location of these restricted RFI bands can be specified in advance before the operation of the multi-carrier transceiver starts, and, for example, the tones in a restricted RFI band permanently disabled during the operation of the transceiver.
RFI can, for example, be one of the many performance limiting factors when a multi-carrier transceiver is deployed in the field. For the multi-carrier transceiver known as ADSL, tests that include measuring the performance of ADSL in the presence of RFI are now being defined in xe2x80x9cG.test.bis: Laboratory Set-ups and procedures to include RFI impairments in the testing of DSL transceiversxe2x80x9d by Nortel Networks(copyright), incorporated herein by reference in its entirety. These tests, as well as other industry-standard tests, provide a good reference model in which the performance RFI mitigation techniques can be measured.
An exemplary embodiment of the present invention describes a multi-carrier information transceiver with robustness against radio frequency interference (RFI) signals present in a communications channel. The multi-carrier transceiver comprises a radio frequency interference mitigation technique that operates, for example, not only during the steady state operation of the transceiver but also during the training state of the transceiver.
The transceiver is able to dynamically modify the training signals when the presence of RFI is detected. For example, the training signals can be modified by dynamically disabling tones in the region of the spectrum where the RFI is detected. For example, this detection can occur during an initialization phase. In this exemplary embodiment, the receiver sends a message instructing the transmitter to disable tones in the multi-carrier signals during certain phases of training and or steady state operation. The message contains, for example, a field that designates which of the tone number(s) are to be disabled and during which stages of training and/or steady state operation they are to be disabled. The transmitter can also receive this message and, for example, disable the specified tones during the specified stages of training and or steady state, for example, during a signal-to-noise ratio measurement and related calculations, during a training of the equalizer, or in other types of training or measurement. During the remaining stages of training and/or steady state, where instructions are not necessarily specified in the message, the transmitter does not disable the specified tones, but could send the standard signals in those tones.
According, an in accordance with an exemplary embodiment of this invention, a first aspect of the invention relates to providing an improved multi-carrier transceiver.
Aspects of the invention also relate to providing a multi-carrier information transceiver in which, for example, prior to the training phase, the presence or absence of RFI in the communications channel can be established. If, for example, RFI is detected, the receiver can instruct the transmitter to disable tones in one or more of the training signals, and during different stages of the modem training phase. The receiver can also instruct the transmitter to disable tones in the information signals during the steady state phase. If no RFI is detected, then, for example, the transmission of both training and steady state signals can occur without disabling any tones.
Aspects of the invention also relate to providing a multi-carrier information transceiver in which a RFI mitigation technique takes advantage of the disabled tones in both the training signals and the steady state signals to better estimate the RFI.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of the embodiments.